I'm going to describe the symptoms with a very short history, some preliminary theories ( based on past work I've done on this car ), the data I've gathered and what, if any, conclusions I might draw from that data. The car is a 98 Mazda 626 GF 2L ATX . This is a European model.

Symptoms

The car had been idling rough, at about 650rpm, which would get significantly worse under load, and it would sometimes ( rarely ) stall at lights. I verified that there were no air leaks, and that fuel trims were normal, so I decided to adjust the idle air screw until it was idling at 750rpm. Once it hit 750, the idle smoothed out, however, any load put on the engine at idle would significantly drop the idle and cause severe shaking. Other than this, there are no performance or drivibility problems that I can detect. The car gets ok, but not great, gas mileage and has plenty of power.After fixing allot of different things on this car, some needed and some not, I've came down to two theories, either a wonky camshaft sensor or a wonky IAC.

Data

So I decided to gather the following data with my scan tool, and with a Hantek 1008C scope. I'll make a note now that I'm a complete novice with a scope, and the Hantek is on top of that a pretty cheap one.The first thing I pulled up was the O2 sensor and fuel trims at idle and just let it run a few minutes. The only load during this time is the radiator fan occasionally coming on.



The thing which really stands out to me here is the lack of consistency in the LTFT. Even though it's in the normal range, it just seems to be wandering all over the place. I then blipped the throttle twice and ran it at about 2k rpms for 20 seconds:



Everything seems pretty normal here, AFAIK.So then I checked the RPM, MAF, IAC Duty Cycle and Spark Advance PIDs with my scan tool while simultaneously checking the the IAC PWM frequency with my Hantek scope:



The WSM says idle should be between 650-750 with MAF reading 2.2 to 3.1 g/s. At idle, the RPMs are slowly oscillating by about 20-30rpm, and the MAF reading between 2.3 to 2.6. The thing that really jumped out at me was the spark advance. It's supposed to be between 6 to 18 degrees at idle, and it's slowly bouncing between 0.75 and 2.75. I verified this with a timing light and tried unplugging the knock sensor, which had no effect. There aren't any knock related codes.Next I started turning loads on at about the 41 second mark. Over about 2 1/2 seconds I turned on the brights, the rear defrost, the AC on high and finally put the car in drive with the parking break on.



As the loads are turned on, the timing advances to 20.5*, while the RPMs bottom out at 492 before recovering to 736. As the RPMs recover, the timing bottoms out to -2.5*, then recovers to 17.5* as the RPMs drop to 597. As the IAC duty cycle slowly increases to about 53% over ten seconds, the timing backs off until it's slowly rolling between about 6.25* and 8.5*.The same scenario plays out when I turn the loads off; the RPMs recover to about 840 and the bottom simultaneously drops out of the timing hitting -3.5* before recovering to 2.5* as the IAC slowly backs off.While I was collecting this data via the scan tool, I was also monitoring the IAC PWM signal with the Hantek. While the scan tool showed the IAC duty cycle PID at 28.5 during no load idle, the scope showed a frequency of about 1.5kHz and a duty cycle of about 33%. When the PID showed a steady 53.91% duty cycle under the loads, the scope showed it oscillating quickly between 574-604Hz and 65% to 67%. .I also took a look at the camshaft signal on the scope. Idling at 750rpm the camshaft spins at about 6.25Hz. While the waveform I saw on the screen seemed OK, the frequency measured by the scope was all over the place, from 5hz up to a few hundred hertz. When I took this screen shot it was at 44Hz:



Not sure if there is actually a problem there, if it's just an error in how I'm measuring, just noise or the scope itself. I'm of course also wondering if my scope IAC measurements are reliable.In addition I scoped the ignition ( waste spark ) and saw to the best of my knowledge good sparks with 2ms burn lines and nice oscillations at the end.So that's my data. There are two codes, both of which are very intermittent and may take from a few days to a few weeks to reappear after being erased. The first is P1500, which is for the VSS sensor. However, there isn't any VSS on this model, and I assume it's getting the data from the ABS wheel speed sensors. The other code is P1632, which is equivalent to P1633 on the US models. The 97 EU WSM doesn't have much useful to say, but the 2001 US manual says:The PCM monitors the voltage of KAM terminal at PCM terminal 55. If the PCM detected B + battery positive terminal voltage is below 10 V for 20 seconds, the PCM determines that the KAM voltage circuit has malfunctioned.If I clear the codes and run the KOEO and KOER self tests they always pass, and everything that's listed to check for "Low, Rough Idle" in the WSM checks out OK.In the past year or two I've replaced the plugs, coil pack, injectors, fuel pump, battery, alternator, water pump and timing belt, TSS sensor, PCV valve grommet, four out of five engine mounts, O2 sensor. I've removed and cleaned the TB, IAC and EGR and seafoamed the intake. Backs of the valves are clean as a whistle, compression is good and cam to lifter clearances are in spec.

Conclusions?

Something is causing the ECU to severely retard the timing, and it doesn't seem to be the knock sensor. While the fuel trims are well within spec, something is causing the LTFT to kind of wander around in the 0 to -5% range at idle.I was originally focusing on the IAC even though it passes a basic resistance test because of what seemed to be a fluctuating MAF reading and due to the report I linked above attributing these symptoms to a worn out IAC. However those timing and LTFT data have thrown me for a bit of a loop and now I'm not sure how to proceed.